Interferometry has been known and studied for many years. In its simplest form optical interferometry involves creating a beam of light which is split into a measuring beam and a reference beam so that the measuring beam is reflected off of the test item and the reference beam is reflected off of a fixed object, the two beams are recombined, and an interference or fringe pattern is created which is proportional to the phase difference imparted on the measuring beam by the surface of the test object. Considerable improvement and efficiency has been achieved over the years, particularly with the development of laser light sources. Monochromatic coherent light allows for greater accuracy and sensitivity in determining minute variations in measuring parameters such as indices of refraction, phase, amplitude, and polarity.
Numerous methodologies exist for analyzing surface characteristics, but all of these have the disadvantage of requiring a specimen to be mounted within an optical interferometer for accurate analysis. There does not yet exist a system capable of on-line precise surface roughness measurement of a product, such as a film, sheeting, or web, during the manufacturing process.
An example of an apparatus and method for surface measuring using phase interferometry is U.S. Pat. No. 3,796,495 issued to Laub on Mar. 12, 1974, which uses laser light to create a measuring beam. The Laub invention uses an acousto-optical modulator of variable frequency to create two measuring light beams which have slightly different frequencies at the optical centers of the two beams. These two beams are then projected onto the surface of the test specimen. The beams have considerable physical space overlap as well as frequency overlap. However, there is a linear zone of interference occurring between the two beams at the surface of the test specimen. These two beams are recombined after reflection off of the test surface to re-create that interference. If the specimen surface is uneven, this linear zone becomes wavy in appearance and is represented on an oscilloscope, at which time a picture is taken and the specimen is moved slightly to get a picture of the next region.
The technique disclosed in the Laub patent is inefficient due to waste of the output light and because only a small portion of the area of beam incidence between the overlapping spots is usable to detect the phase displacement, or shear, between the frequency differentiated measuring beams. Furthermore, variability in the ramp generator to the acousto-optical modulator leads to fluctuations in the distance between optical centers of the two measuring beams. This will artificially introduce phase differences as the beam centers shift away from and toward each other. The Laub method is subject to temperature and vibrational contamination as well as fluctuations that may be caused by changes in the focal length and out of focus measurements.
Another example of a multiple beam interferometer is disclosed in international patent application WO92/0397, published Mar. 5, 1992. This application discloses an interferometer using two or more first-order beams derived from an acousto-optical modulator driven at two or more frequencies. The two or more beams are focused onto the test surface and on reflection from the surface are recombined with a separate reference beam to create the interference. The two measuring beams are electronically switched back and forth so that only one beam at a time is directed to the test surface. Despite the use of two measuring beams on the test surface, this apparatus is essentially two superimposed and parallel Michelson interferometers. This apparatus is less susceptible to vibration as compared to a single Michelson interferometer but it does not eliminate vibrational noise because of its reliance on a separate stationary reference beam to create the interference. Other difficulties with this apparatus relate to the use of two or more beams differentiated by frequency, which requires intervening electronic switching between measurements from one beam to the next.
A third example of a multiple beam interferometer is disclosed in U.S. Pat. No. 5,139,336 issued to See et al. on Aug. 18, 1992. The patent discloses a heterodyned interferometer using amplitude modulation as well as different frequencies between the two measuring beams. The device either uses a zero order and one first-order beam for measuring a surface or it incorporates at least two driving frequencies for a Bragg cell acousto-optical modulator to create two first-order beams of different frequency. In either case the measuring beams undergo amplitude modulation. As disclosed, this device is affected by surface color and reflectivity changes.
There does not yet exist a system and method teaching on-line interferometry for use during a manufacturing process to control that manufacturing process. Difficulties due to vibration within the system, movement of the test specimen, color, chemistry, reflectivity changes, changes in amplitude, background electronic noise, and temperature fluctuation can all introduce significant error into the measurements obtained by an interferometer. Available systems do teach methods and systems that are able to control these variables, but do so by removing test specimens from a manufacturing process. Each specimen is then tested in a controlled environment and may not produce reliable test results representative of the entire amount of product from which the sample is taken.